1. Field of the Invention
The present invention relates to a method of controlling a printer, and more particularly, to an efficient tone reproduction curve (TRC) control method for providing images of high quality in response to environmental changes. The present application is based on Korean Patent Application No. 2001-48522, filed Aug. 11, 2001, which is incorporated herein by reference.
2. Description of the Related Art
A usual electrophotographic process applied in printers includes charging a photosensitive body with electricity, exposing the charged photosensitive body to form a latent image in a particular image area, adhering a developer to an area of the latent image on the charged photosensitive body using a developing device, transferring the developed image to a sheet, and fixing the image using a fusing roller.
In the charging process, the quality of the image can be increased by uniformly charging the photosensitive body with electricity. Accordingly, it is necessary to control the electric potential of the photosensitive body to be uniform during the charging. When the charging potential is low, contamination can occur in a non-image area. When the charging potential is high, developed mass per area (DMA) changes. When the charging potential is excessively high, the photosensitive body is permanently damaged.
Since the quality of an output image is related to the potential of a photosensitive body, it is necessary to maintain the potential of the photosensitive body within a predetermined range in order to obtain images of high quality. An electrostatic voltmeter (ESV) or an electrometer is used for measuring the potential of the photosensitive body. The ESV is disposed near the surface of a photoreceptor belt so that it can measure the potential of the photosensitive body when the photoreceptor belt passes an electrostatic electrode.
The potential of the photosensitive body is decreased to a predetermined potential referred to as an exposure potential to form the latent image during the exposure. The development device comes to have a development potential such that the potential of the developer is higher than the potential of a portion on which the latent image is formed, that is, the exposure potential, and lower than the potential of the other portion of the photoreceptor belt on which the latent image is not formed. As a result, the developer is made to adhere to an area of the latent image and thus development is performed.
The DMA of the developer during development is influenced not only by the charging potential, as described above, but also by the exposure potential and the development potential.
When the exposure potential is low, the difference between the exposure potential and the development potential is very big even if the development potential is maintained uniform. As a result, the amount of absorbed developer increases. When the exposure potential is high, the difference between the exposure potential and the development potential is small even if the development potential is maintained uniform. As a result, the amount of absorbed developer decreases, thereby producing a blurred image.
Similarly, under the condition that a constant charging potential and a constant exposure potential are applied to the photosensitive body, when a development potential applied to the photosensitive body is very high, the small difference between the development potential and the exposure potential causes the developer to be excessively absorbed. In contrast, when a development potential applied to the photosensitive body is very low, the big difference between the development potential and the exposure potential causes the developer to be poorly absorbed, thereby blurring an image.
FIG. 1 shows a method of controlling DMA to correct development errors in order to obtain an image of high quality, which is disclosed in U.S. Pat. No. 5,749,021. According to the method, the charging potential, exposure potential, and development potential are controlled based on internal process parameters known as a discharge ratio, a cleaning potential and a development potential.
This conventional method increases the quality of a printed image by controlling DMA in a process control loop. An area in which an image is formed is referred to as an image area. A test patch is usually provided between image areas to measure DMA. The measured DMA is compared with a target value, an error signal is transmitted to a controller, and the internal process parameters are adjusted, thereby correcting errors. In other words, a grid potential and an average beam power of an exposure system are calculated using the internal processes parameters to control the system of a printer.
Referring to FIG. 1, a level 1 controller 120 provides suitable control signals Ug and Ul to an electrostatic charging and exposure system 122 to control the electrostatic charging and exposure system 122. Reference numeral 124 denotes a charging potential value Vh and an exposure potential value Vl of the electrostatic charging and exposure system 122, which are measured by an ESV. Comparators 126a and 126b compare the values Vh and Vl with target values VhT and VlT denoted by reference numeral 128 for a charging potential and an exposure potential, respectively, and transmit error signals Eh and El, respectively, denoted by reference numeral 129 to the level 1 controller 120. The gain of a level 1 loop is obtained from the error signals such that the potentials of a photosensitive body can converge to target values within a predetermined range.
The target values VhT and VlT for the charging potential and exposure potential provided to the level 1controller 120 and the electrostatic charging and exposure system 122 are generated from a level 2 controller 130. Comparators 136a, 136b, and 136c compare DMA sensor values Dl, Dm, and Dh denoted by reference numeral 134, which are measured from test patches provided according to a toner area coverage in a color toner density (CTD) sensor, with target values DlT, DmT, and DhT denoted by reference numeral 138, respectively, and transmit error signals 139 to the level 2 controller 130. In addition, the level 2 controller 130 generates a signal VTd for controlling a development system 132.
In other words, in the conventional DMA control method, DMA values measured by a CTD sensor are compared with target DMA values to generate differences therebetween. The differences are transmitted to a level 2 controller. The level 2 controller linearizes internal process parameters, i.e., a discharge ratio, a cleaning potential, and a development potential and extracts target values for control parameters, i.e., a charging potential, an exposure potential, and a development potential, from the linearized discharge ratio, cleaning potential and development potential to control a level 1controller and charging, exposure, and development systems.
The conventional DMA control method disclosed in U.S. Pat. No. 5,749,021 uses not only a CTD sensor but also an electrostatic sensor in order to diagnose the status of all parameters influencing an electrophotographic process, which complicate measurement. In addition, the conventional DMA control method is not adaptive in a state in which a charging, exposure, or development system changes due to changes in external environmental parameters such as temperature and humidity of a printer, or due to internal environmental changes such as replacement or supplement of substances such as a developer or a photosensitive body included in the printer. Moreover, the conventional DMA control method has a disadvantage of individually linearizing a discharge ratio, a cleaning potential, and a development potential in order to control a charging potential, an exposure potential, and a development potential based on internal process parameters, i.e., a discharge ratio, a cleaning potential, and a development potential.
To solve the above-described problems, it is a first object of the present invention to provide a toner reproduction curve (TRC) control method for providing an image of high quality in response to external and internal environmental changes.
It is a second object of the present invention to provide a simple TRC control method for diagnosing the state of a printer.
To achieve one of more objects of the invention, there is provided a method of controlling a TRC in a printer including a color toner density (CTD) sensor receiving light reflected from test patches having different densities and photoelectrically converting the received light, the test patches being provided on a photoreceptor belt. The method includes the steps of (a) measuring a development potential VB, a charging potential VO, an exposure potential VR, and a development current Id and evaluating a development vector VD, which is the difference between the development potential VB and the exposure potential VR, and a backplating vector VBP, which is the difference between the charging potential VO and the exposure potential VR; (b) forming a TRC space using TRC data detected from the CTD sensor; (c) obtaining a TRC characteristic function from the TRC data, the development vector VD=x, the backplating vector VBP=y, and the development current Id=z; (d) forming an RTRC by setting TRC data, whose covariance is smaller than a threshold value among the detected TRC data, as RTRC data; and (e) measuring TRC data, comparing the measured TRC data with the RTRC data to calculate control parameter values, and controlling the printer using the control parameter values.
The step (a) includes the steps of (a-1) measuring a development potential VB, a charging potential VO, an exposure potential VR, and a development current Id; and (a-2) evaluating a development vector VD and a backplating vector VBP from the measured development potential VB, charging potential VO and exposure potential VR. The development vector VD and the backplating vector VBP satisfy the following formulae.
VD=VBxe2x88x92VRxe2x80x83xe2x80x83(1)
VBP=VOxe2x88x92VBxe2x80x83xe2x80x83(2)
The step (b) includes the steps of (b-1) developing a test patch at a high toner area coverage, a test patch at a mid toner area coverage, and a test patch at a low toner area coverage on the photoreceptor belt and detecting a signal TH corresponding to the high toner area coverage, a signal TM corresponding to the mid toner area coverage, and a signal TL corresponding to the low toner area coverage from the CTD sensor receiving infrared rays reflected from the test patches and generating electrical signals; and (b-2) forming a TRC space using the detected signals TH, TM, and TL.
The step (c) includes the steps of (c-1) obtaining a non-linear equation satisfying KVxe2x88x92T=0; (c-2) obtaining a Jacobian matrix J of the non-linear equation; (c-3) deducing the values of x, y, and z by combining an equation obtained from the Jacobian matrix J and the non-linear equation with the detected TRC(TH, TM, TL) data.
The step (d) includes the steps of (d-1) deducing a development vector VD, a backplating vector VBP, and a development current Id by combining the TRC(TH, TM, TL) data detected in step (b-1) with the TRC characteristic function obtained in step (c); (d-2) forming an RTRC(TRH, TRM, TRL) space using TRC(TH, TM, TL) data, whose covariance is smaller than the threshold value, among the TRC(TH, TM, TL) data detected in step (b-1); and (d-3) determining a function having the development current Id as an independent parameter and the development vector VD as a dependent parameter and a function having the development current Id as an independent parameter and the backplating vector VBP as a dependent parameter by curve fitting the development vector VD, the backplating vector VBP, and the development current Id to be suitable to the RTRC(TRH, TRM, TRL) space formed in step (d-2).
The step (e) includes the steps of (e-1) measuring TRC(TH, TM, TL) data in real time; (e-2) comparing the measured TRC(TH, TM, TL) data with RTRC(TRH, TRM, TRL) data; (e-3) calculating a development vector VD and a backplating vector VBP with respect to the measured development current Id from the TRC characteristic function obtained in step (c), when the deviation between the measured TRC(TH, TM, TL) data and the RTRC(TRH, TRM, TRL) data is greater than a tolerance error; (e-4) calculating a new development vector VDxe2x80x2 and a new backplating vector VBPxe2x80x2 by combining the measured development current Id with the functions of the development vector VD and the backplating vector VBP obtained in step (d-3); (e-5) calculating a value of a grid potential VG and a value of a development potential VB as the control parameter values using the new development vector VDxe2x80x2 and the new backplating vector VBPxe2x80x2 calculated in step (e-4); and (e-6) controlling the charging potential VO, exposure potential VR and development current Id of the printer by applying the value of a grid potential VG and the value of a development potential VB to the printer.
Preferably, the step (e) further includes the step of (e-7) estimating the efficiency of a charger from the development vector VD and the backplating vector VBP which are calculated in step (e-3) and from a development potential VB and a grid potential VG which are measured.
The present invention controls a grid potential and a development potential by deducing the potentials (charging potential, exposure potential, and development potential) and development current of a photosensitive body which are necessary for maintaining and controlling the TRC of an apparatus such as an electrophotographic printer or a copy machine which uses an electrophotographic process.
Particularly, the potentials (charging potential, exposure potential, and development potential) of a photosensitive body are deduced using TRC data measured by a CTD sensor, without using an electrostatic sensor for measuring the charging potential and exposure potential of the photosensitive body and a sensor for measuring the development current. Here, internal process parameters are a development vector, a backplating vector, and a development current which are defined above. A target RTRC value is set based on the result of deduction so that the TRC can approximate an RTRC to maintain or control the TRC.